1. Field of the Invention
The present invention relates to the frequency offset estimating technology, and it particularly relates to a frequency offset estimating method for estimating frequency offsets contained in signals received by a plurality of antennas and a frequency offset correcting apparatus utilizing said method.
2. Description of the Related Art
In wireless communication, it is generally desired that the limited frequency resources be used effectively. One of the technologies that effectively utilize the frequency resources is adaptive array antenna technology. In the adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. In other words, the apparatuses provided with adaptive array antennas change respectively the amplitude and phase of signals received by the plurality of antennas, sum up respectively a plurality of the thus changed received signals, and receive the signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in said amplitude and phase (hereinafter referred to as “weight”). In addition, signals are transmitted in a directional pattern of the antenna corresponding to the weight.
One example of processings to compute the weights in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula whose amount of calculation is smaller than directly solving the Wiener solution. For such recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used.
However, frequency offsets are present between carriers outputted from a local oscillator included in a transmitting apparatus and those outputted from a local oscillator included in a receiving apparatus, thus causing to generate phase errors. When QPSK (Quadrature Phase Shift Keying) is used as a frequency modulation scheme, the constellation of signals received by the receiving apparatus is rotated by phase error. In general, the rotation of constellation like this degrades the transmission quality of signals. When an adaptive algorithm is used in adaptive array antenna technique, there are some cases where the frequency offsets can be estimated in a form such that the frequency offsets are contained in the weight (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Japanese Patent Application Laid-Open No. Hei10-210099.
When the weight is to be calculated by using LMS algorithm as the adaptive algorithm, the frequency offsets can be also calculated in a form such that the frequency offsets are contained in the weight. However, the range in which the frequency offset can be calculated will be narrow in general. That is, the larger the frequency offset becomes, the harder the accurate estimation of said frequency offset will be. In addition, if the number of weights, namely, the number of antennas increases, the range in which the frequency offset can be calculated will be further narrowed. As one method, on the other hand, for broadening the range in which the frequency offset can be calculated using LMS algorithm, the method may be such that the step-size parameter of LMS algorithm is made small. According to this method, the filtering effect is small in general, thus resulting in the drop of signal transmission quality. When the multi-carrier scheme is used to transmit signals, the frequency offset gives rise to intercarrier interference, thus resulting in the further drop of signal transmission quality.